Compressed air storage containers/pressure vessels/air tanks are employed for the storage of air at elevated pressures. These air tanks are widely employed to enable the functionality of various mechanisms, such as pressure washers and air driven tools, and may commonly be utilized in the consumer market, residing in consumer's shops, garages, and barns. A typical problem associated with the use of these compressed air tanks is that due to humidity in the air and temperature changes moisture (i.e., water) may build up within the tank which may lead to corrosion of the tank, if the tank is not properly maintained by regularly draining the condensate from the tank. Compressed air tanks commonly include drain plugs which allow the operator to perform proper maintenance draining. However, it may be the case that operators may fail to properly maintain the air tanks, thereby, resulting in an amount of condensate residing in the air tank for an extended period of time. This water may lead to corrosion of the inside of the air tank with no indication of damage viewing the air tank from the outside. The corrosion of the air tank may lead to a rupture of the tank walls, which may result in a decreased useful life span and serious damage to the air tank.
Compressed air tanks may be composed of various materials. Steel is often employed in the construction of these air tanks. Steel corrosion by water is typically described by a single corrosion rate, usually millimeters (or thousandths of an inch) per year. This corrosion is generally thought to be uniform (i.e., the same at all points on the corroding surfaces.) However, steel corrosion is often not uniform and may have pits or other localized corrosion.
For pitting corrosion, the corrosion rate is measured at the maximum pit depth and the rate can be as high as three times the uniform corrosion rate for the same material under the same environmental conditions. Severe pitting may lead to a leak while some structural strength remains. Thus, for a corroding compressed air tank of an air compressor, pitting may lead to leaks before a rupture may occur. This is referred to as “leak-before-burst”.
Another form of non-uniform corrosion is known as “waterline attack”. In this corrosion process, the corrosion rate is greater at the splash zone or the intersection of the metal, the corroding liquid, and air. This may result in a “line”, commonly referred to as a “waterline” in the air tank that is a thinner or weaker section of the air tank than the surrounding area. A waterline, long length of thin metal, aligned perpendicular to the maximum stress direction, the hoop stress direction, may result in a rupture well before an expected rupture based on metal thickness in the area of uniform corrosion.
Compressed air tanks are typically assembled from formed flat sheets of steel that are welded together. The welding process alters the local metal structure in and near the weld, establishing a fusion region. This may lead to non-uniform corrosion of the metal in or near the fusion region. The fusion region of the weld (metal that was molten) has more microstructure variation than wrought metal and may be prone to pitting. There is a region near the weld that does not melt, but gets hot enough to alter the metallurgical structure. This is called the heat-affected zone (HAZ). This different metallurgical structure may cause local corrosion attack. If local corrosion occurs at the HAZ it may be manifested as a line of thin metal next to the weld. Like waterline attack, localized corrosion in the HAZ may lead to a rupture before one is predicted using expected metal thickness estimated from uniform corrosion.
Currently, many available compressed air tanks employ a method commonly referred to as a “telltale hole” for assisting an operator in identifying internal tank corrosion. This method typically entails mechanically thinning the wall of the tank with a single, small diameter “telltale hole”. The telltale hole partially penetrates the wall of the tank. The partial penetration hole established is cut from the outside and when the remaining metal of the tank wall corrodes away, the tank leaks at the hole to warn that the tank's useful life has expired. Unfortunately, this method assumes that the first place a tank will leak due to corrosion will be at the “telltale hole” and that when a leak occurs at the “telltale hole”, rupture of the tank will generally be averted because there is still enough metal surrounding the hole and in the rest of the tank to support the pressure and avoid rupture or bursting.
The problem with this method is that the location for drilling the “telltale hole” is typically at the bottom of the tank and may not accurately reflect the degree of corrosion that is occurring further up the wall of the tank, from the bottom of the tank up to and including the “waterline” and above. The “waterline” being the point of intersection of the metal of the tank, corroding liquid within the tank, and remaining air within the tank. This may be problematic because there are occasions where the corrosion rate may be much greater at various locations up the tank wall, such as at the “waterline”, than at the point where the “telltale hole” is drilled. When the rate of corrosion is greater at the “waterline”, this is commonly referred to as “waterline attack” as stated previously, and may result in a catastrophic rupture of the tank at the “waterline”, well before a leak occurs at the “telltale hole”.
Therefore, it would be desirable to provide an apparatus enabled to detect corrosive effects occurring within a compressed air tank at various locations within the compressed air tank.